NTSC

Background

NTSC stands for the "National TelevisionStandards Committee". Though the committee is responsible for more standards than the one described below, their analog color broadcast standard, it is the one they are most famous for.

As early as 1929, people were experimenting with ways to transmit a color TV signal. After the FCC rejected RCA's Dot-Sequential Color System proposal in 1950, the current NTSC standard was developed with industry-wide cooperation. A major goal of this system was to be backwards compatible with existing black-and white TV sets, unlike some of the earlier proposals. It was submitted as a petition to the FCC in July 1953, and accepted that December. While the standard has served well since, the analog broadcasts are scheduled by the FCC to end in 2006, when the associated licenses will be revoked in favor of newer digital standards.

Despite its years of service, the NTSC standard isn't highly regarded by video engineers. A couple of less favorable meanings for the NTSC acronym have cropped up through time. "Never The Same Color" (sometimes seen as "Never Twice the Same Color") is explained below, and "Never Tested Since Christ" is also common.

The video signal itself is a composite of three channels in the YIQcolor space. The color subcarrier is at 3.58 MHz above the video carrier, or 4.83 MHz into the entire band. The 3.58 MHz number was chosen to minimize interference with the luminance: it is the 455th half-harmonic of the horizontal line frequency. That is, 3.58 MHz = 455/2 * 15734 Hz.

Y (Luminance): AM brightness signal, spanning the whole bandwidth, which carries the signal to the black-and-white sets as well. It uses vestigial sideband encoding instead of single or double sideband, sending a full 4.2 MHz upper sideband and the lower 750 KHz of the lower sideband. This allows it to carry the most important, low-frequency information better.

I (Orange-Cyan balance): QAM signal covering from -1.5 MHz to +0.5 MHz from the color subcarrier, sent as double sideband.

Q (Green-Magenta balance): QAM signal covering half the bandwidth of I, from -0.5 to +0.5 MHz from the subcarrier, also sent as double sideband.

The reasons behind the bandwidth reductions for the color signals are that human vision is more sensitive to changes in brightness than color, so that less resolution is required for the color information. Similarly, I and Q were chosen instead of U and V because humans are less sensitive to green-magenta changes than other color axes, allowing for further reduction of that signal.

Since the chrominance information isn't synchronized with anything else, an 8-10 cycle colorburst is transmitted on the back porch. The colorburst itself is simply the raw, unmodulated carrier for the receiver to synchronize with before the next line.

Unlike PAL/SECAM, NTSC never alternates the phase of the color carrier, so the signal is subject to phase drift under certain conditions. Since the hue is encoded by the phase of the color signal, this causes distortion of the colors. NTSC TV sets are therefore equipped with a manual phase adjustment, the Tint control, to correct for any phase drift. This feature is what earned NTSC the reputation of meaning "Never The Same Color".

The image itself is interlaced, with two fields composing each frame, and each field carrying 262 lines of the final 525-line image. Of those 525 lines, 480 are usable for display, and the rest compose the vertical blanking interval, or VBI. In the beginning, the VBI was blank, but later developments have added information into it, notably captions for closed captioning of broadcasts.

Sources:
Technical:
http://www.filmbug.co.uk/dictionary/pal-ntsc.php
http://cnyack.homestead.com/files/modulation/ntsc_sig.htm
History:
http://www.novia.net/~ereitan/NTSC_overview.html
http://nemesis.lonestar.org/reference/internet/web/color/ntsc_primer.html
Countries of use were taken from xmatt's original writeup, with corrections by althorrat.